[rrd-users] Forcing our own dates?

[Steve Shipway]

> is it possible to retrieve the date from the db as part of an overall
> query as opposed to using the current system date for the plotting of a
> graph. For example say I have a join that will show me the following:

I think you might be misunderstanding the nature of the RRD.  It is not a relational database, and does not have tables and records; it does not take SQL queries (although some people have made a SQL interface to it, the structure of the RRD does not lend itself to SQL readily).

Since the RRD contains 'RRA's which are roll-ups of posted data points and expired automatically over time, it is increasingly unlikely that you can retrieve a single data point value.  You will also not be able to retrieve the exact insert time due to the 'data normalisation' that brings the insert time in line with the 'time buckets' (eg rounding off to the next multiple of 5min).  Finally, there is no 'table name' or 'db name' as such, though 'RRA' and 'RRD file' might be analagous.

For someone coming from a structured relational-database background, the RRD might appear to be horribly limited.  However, it is a highly specialised database structure optimised for a specific purpose which it performs very efficiently -- it should not be discounted because it cannot perform functions it was never intended for.

When making a graph generation call, you can of course pass any times you want for the start and end points (although whether or not the data are available in one of the RRAs at the required resolution is another matter).

Steve

Steve Shipway
ITS Unix Services Design Lead
University of Auckland, New Zealand
Floor 1, 58 Symonds Street, Auckland
Phone: +64 (0)9 3737599 ext 86487
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Email: s.shipway at auckland.ac.nz
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